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Diurnal variation in Sahara desert sand emissivity during the dry season from IASI observations
Author(s) -
Masiello Guido,
Serio Carmine,
Venafra Sara,
DeFeis Italia,
Borbas Eva E.
Publication year - 2014
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/jgrd.50863
Subject(s) - emissivity , diurnal temperature variation , depth sounding , environmental science , atmospheric sciences , diurnal cycle , planetary boundary layer , water vapor , remote sensing , geology , meteorology , geography , physics , optics , turbulence , oceanography
The problem of diurnal variation in surface emissivity over the Sahara Desert during non‐raining days is studied and assessed with observations from the Infrared Atmospheric Sounding Interferometer (IASI). The analysis has been performed over a Sahara Desert dune target area during July 2010. Spinning Enhanced Visible and Infrared Imager observations from the European geostationary platform Meteosat‐9 (Meteorological Satellite 9) have been also used to characterize the target area. Although the amplitude of this daily cycle has been shown to be very small, we argue that suitable nighttime meteorological conditions and the strong contrast of the reststrahlen absorption bands of quartz (8–14 μm) can amplify its effect over the surface spectral emissivity. The retrieval of atmospheric parameters show that, at nighttime, an atmospheric temperature inversion occurs close to the surface yielding a thin boundary layer which acts like a lid, keeping normal convective overturning of the atmosphere from penetrating through the inversion. This mechanism traps water vapor close to the land and drives the direct adsorption of water vapor at the surface during the night. The diurnal variation in emissivity at 8.7 μm has been found to be as large as 0.03 with high values at night and low values during the day. At 10.8 μm and 12 μm, the variation has the same sign as that at 8.7 μm, but with a smaller amplitude, 0.019 and 0.014, respectively. The impact of these diurnal variations on the retrieval of surface temperature and atmospheric parameters has been analyzed.